TCP has a parameter called retransmission time-out existing and when a packet is abandoned on a path in a network, a TCP stack retransmits the packet after waiting for a lapse of a retransmission time-out. TCP stack is in general mounted on OS (Operating System).
Since OS is mounted in software here, retransmission time-out of TCP cannot be determined with high precision. For example, since a unit time (Jiffies) of Linux is on a 10-millisecond basis, a standard TCP stack of Linux is not allowed to have a retransmission time-out reduced to be less than 10 milliseconds.
In a network whose RTT (Round Trip Time) is extremely short such as a data center, by reducing a retransmission time-out down to be of the order of microseconds, FCT (Flow Completion Time) is decreased to improve communication efficiency. Since a TCP stack operable on OS is not allowed to have the retransmission time-out reduced down to a microsecond level for the above-described reason, FCT cannot be reduced in such a network.
Proposed for solving the problem is such a packet retransmission control system as disclosed in Non-Patent Literature 1 as related art. According to the packet retransmission control system recited in the Non-Patent Literature 1, packet retransmission is controlled by executing time-out determination on a microsecond level in a MAC layer lower than a TCP layer. This enables FCT to be reduced in a network whose RTT is short and improves communication efficiency without changing a structure of OS or a TCP stack.    Non-Patent Literature 1: Toru Takamichi, Masahiro Shigihara and Kiyoshoi Fukuchi, “Ultra-High Speed MAC Technology for Terabit-Class LAN”, Proceedings of the 2008 IEICE Society Conference.
First problem of the packet retransmission control technique according to the related art is that an effect of retransmission control of a MAC layer can be obtained only in a short flow.
The reason is that since at the time of long flow data transfer, a fast retransmission function of TCP is started by three or more successive overlapping TCP-ACK (Acknowledgement) packets, a missing packet will be retransmitted without waiting for retransmission time-out.
As a result, a packet is retransmitted on the TCP level without waiting for retransmission time-out in the packet retransmission control technique, so that none of the effects of the packet retransmission control technique can be obtained in a long flow.
Short flow denotes a flow whose transfer data size per TCP flow is small. More specifically, it denotes a flow formed of one or two packets. In contrast with a short flow, long flow denotes a flow whose transfer data size per TCP flow is large.
On the other hand, in a case of a short flow, three or more successive overlapping ACK fail to occur which trigger fast retransmission. Accordingly, even when packet abandonment occurs, retransmission of a missing packet by TCP will not be executed until retransmission time-out of TCP.
Second problem is an increase in a packet abandonment rate when congestion occurs in a long flow of TCP.
The reason is that even when a packet is abandoned at a switch (network switch) on a path, a missing packet will be retransmitted in a MAC layer by the packet retransmission control technique, so that the TCP layer fails to sense abandonment of the packet and therefore, no TCP congestion algorithm works even when congestion occurs, which prevents a decrease in a sending rate.